This invention relates to co-channel interference reduction in wireless communications systems, for example cellular radio systems using TDMA (time division multiple access) techniques, such as so-called IS-54, IS-136, and GSM systems.
An important factor that limits the performance of cellular radio communications systems is the existence of co-channel interference, typically from reuse of the same frequency bands and time slots in different cells of the system. The significance of co-channel interference increases with increasing requirements for communications capacity of the system.
It is well known that co-channel interference limitations can be reduced by the use of multiple antennas or antenna arrays. However, this has significant disadvantages in that it also requires the use of multiple RF (radio frequency) front ends or receiver stages, one for each antenna, resulting in excessive costs. In practice, for the reverse link or upstream direction of transmission from a typically mobile end station to a base station of a TDMA cellular radio communications system, in practice it is desirable for the base station receiver to use only two antennas.
It is also known to use DMI (direct matrix inversion) techniques to process base station received signal samples to determine a linear combination or weighting that is intended to minimize the MSE (mean square error) between the combined output and the transmitted signal, this determination being achieved by a Wiener weight solution during each TDMA time slot when the transmitted signal data sequence is known, e.g. during the transmission of known synchronization (e.g. SYNC) and/or colour code (e.g. CDVCC) signals. With only two antennas, only two weights are required, and a 2xc3x972 matrix inverse for the Wiener solution is relatively simple to implement.
Using DMI techniques, the optimum weights which are determined and fixed at one part of the time slot can become quite inappropriate at other parts of the time slot, so that such techniques tend to be inadequate under fast and frequency-selective fading conditions, as typically occur with mobile cellular radio communications systems.
Furthermore, the use of only two antennas, providing only two received signal sample sequences, only allows for the nulling or rejection from the desired signal of a single co-channel interference signal. Typically in a cellular radio communications systems there may be two or three simultaneous independent co-channel interference signals, which may be of similar signal strengths. When multiple interference signals are present, with a known system using only two antennas only the strongest interference signal is rejected from the desired signal, and the presence of the other interference signals can result in high error rates in the desired signal.
An object of this invention is to provide an improved method of and apparatus for reducing co-channel interference.
One aspect of this invention provides a method of reducing co-channel interference in a receiver arrangement of a communications system providing two received signals each comprising transmitted symbols having a predetermined symbol rate, comprising the steps of: for each received signal: sampling the received signal to provide a sampled received signal comprising symbols at twice the predetermined symbol rate; and deriving first and second sampled signals from the sampled received signal, each of said first and second sampled signals comprising samples at the predetermined symbol rate corresponding to respective alternate samples of the sampled received signal at twice the predetermined symbol rate; combining the first and second sampled signals derived from the two received signals with respective weights to produce an output signal; and determining the respective weights to reduce co-channel interference with a desired signal represented by said output signal.
Preferably the step, for each received signal, of deriving the first and second sampled signals comprises decimating by a factor of two samples of the sampled received signal at twice the predetermined symbol rate to produce the first sampled signal, delaying samples of the sampled received signal at twice the predetermined symbol rate by one symbol to produce a delayed sampled received signal, and decimating by a factor of two samples of the delayed sampled received signal at twice the predetermined symbol rate to produce the second sampled signal.
The step of combining the first and second sampled signals derived from the two received signals with respective weights to produce the output signal can comprise multiplying each of the first and second sampled signals by a respective weight to produce a respective product, and summing said respective products. This step can further comprise delaying each of the first and second sampled signals by at least one symbol at the predetermined symbol rate to produce at least one respective delayed signal, multiplying the respective delayed signals by respective weights to produce further products, and summing the further products with said respective products to produce the output signal.
Another aspect of this invention provides a receiver arrangement for a communications system, comprising: first and second receivers for providing two received signals each comprising transmitted symbols having a predetermined symbol rate; samplers for sampling the received signals at twice the predetermined symbol rate to produce sampled received signals; delay elements for delaying the sampled received signals each by one symbol at twice the predetermined symbol rate to produce delayed signals; decimators for decimating the sampled received signals and the delayed signals each by a decimation factor of two to produce four sampled signals each comprising samples at the predetermined symbol rate; and a combining arrangement for combining the four sampled signals with respective weights to produce an output signal.
The combining arrangement can comprise a linear filter for each of the four sampled signals. Such a linear filter can comprise at least one delay element for each of the four sampled signals for delaying the respective sampled signal by one symbol at the predetermined symbol rate, at least two multipliers for multiplying the respective sampled signal and an output of each delay element by a respective weight to produce a respective product, and a summing circuit for summing the respective products to produce the output signal.
Each of the first and second receivers can comprise an antenna and a radio frequency circuit for providing a respective one of the two received signals, and can further comprise a matched filter for filtering the respective received signal in accordance with a function g(xe2x88x92t), where g(t) represents a signal pulse shaping function applied to pulses of a transmitted signal of the communications system. For example, the signal pulse shaping function can have a square root raised cosine frequency response with a predetermined roll-off factor, for example about 0.35.
The invention also provides a method of reducing co-channel interference with a desired signal in a receiver arrangement of a TDMA cellular radio communications system having two antennas and receivers for providing two received signals, comprising deriving two sampled signals from each of the two received signals by sampling the respective received signal at a rate of at least twice a predetermined symbol rate of the system and separating alternate samples at twice the predetermined symbol rate to produce said two sampled signals each with different samples at the predetermined symbol rate, linearly combining the two sampled signals derived from each of the two received signals with respective weights to produce an output signal, and determining the respective weights to reduce co-channel interference with the desired signal represented by said output signal.